This application is a 371 of PCT/FR00/02909 Oct. 19, 2000.
The present invention relates generally to a process for preparing a thiophene derivative.
More particularly, the invention relates to a process for preparing 2-thienyl-2-ethanol of formula: 
This compound has been found to be advantageous as a synthetic intermediate for preparing various chemical products, especially medicinal products derived from [3,2-c]thienopyridine, which are useful as platelet aggregation inhibitors and antithrombotic agents.
Among the [3,2-c]thienopyridine derivatives endowed with such properties, mention may be made in particular of 5-(2-chlorobenzyl)-4,5,6,7-tetrahydro[3,2-c]thienopyridine or ticlopidine (DCI) and also methyl xcex1-(4,5,6,7-tetrahydro[3,2-c]thieno-5-pyridyl)(2-chlorophenyl)acetate, more particularly in the form of its dextrorotatory enantiomer, or clopidogrel.
Various processes for the industrial preparation of 2-thienyl-2-ethanol are known. One of the most widely used is without doubt the process described in U.S. Pat. No. 4,127,580 involving the metallation of thiophene using butyllithium, the treatment of the 2-lithiated derivative thus obtained with ethylene oxide and the hydrolysis of the compound formed to give the desired compound.
However, this process is not free of drawbacks on account, firstly, of the production of butane that is inherent to the use of butyllithium, and secondly the relatively high cost of this metallating agent, which proportionally represents a large fraction of the cost price of the [3,2-c]thienopyridine derivatives finally obtained.
The preparation of 2-thienyl-2-ethanol in an industrially viable process capable of avoiding the drawbacks of the above prior process remains of unquestionable interest.
Various examples of the metallation of thiophene starting with an alkali metal have been reported in the chemical literature.
For example, the preparation of 2-thienylcarboxylic acid by transient metallation of thiophene using the naphthalene-sodium complex, followed by carbonatation of the sodium derivative to obtain the acid in question, has been disclosed.
However, the yield of acid thus formed was only 40%. An entirely similar reaction was reported in J. Chem. Soc. Perkin II (1974), 745-748. However, in the described process, the sodium was replaced with lithium to give, after carbonatation, 41% of 2-thiophenecarboxylic acid.
However, this article mentions a modification to the lithiation process described, in which the thiophene is metallated with the lithium-naphthalene complex in the presence of 1,1-diphenylethylene or xcex1-methylstyrene to give, after carbonatation, 2-thiophenecarboxylic acid in yields of at least 77% or even greater than 90%.
It has now been found, surprisingly, that 2-thienyl-2-ethanol can be obtained in very high yields of the order of 80% by intermediate sodation of thiophene using sodium and xcex1-methylstyrene with the exclusion of naphthalene.
Thus, according to the invention, 2-thienyl-2-ethanol is prepared according to a process involving the following steps:
a) metallation of thiophene using an alkali metal, in the presence of an electron transfer agent,
b) treatment of the compound thus obtained with ethylene oxide,
c) hydrolysis of the thienyl derivative thus formed to give the desired 2-thienyl-2-ethanol.
The alkali metal used in the process of the invention may be lithium, sodium or potassium. However, sodium is a particularly preferred alkali metal.
This alkali metal is usually and preferably used in the form of a dispersion of the finely divided metal in a medium that is not electron-donating, the size of the sodium particles thus divided ranging from 1 to 100 microns, more generally from 1 to 30 microns and preferably from 1 to 10 microns.
Such sodium dispersions can be obtained by heating a mixture of sodium in a suitable medium to a temperature above 97.5xc2x0 C., to give a binary system of immiscible liquids that may be emulsified in the same manner as water and oil. Subsequently, when this emulsion is cooled below this temperature of 97.5xc2x0 C., the sodium solidifies in the form of minute spheroids in suspension in the medium under consideration.
Consequently, sodium dispersions can be prepared by heating a mixture of sodium metal in a suitable medium, to beyond the melting point of sodium, and by emulsifying the whole by very rapid stirring. The medium must have a boiling point higher than the melting point of the metal, unless the dispersion is prepared at a pressure above atmospheric pressure. In this case, it may be envisaged to use thiophene-both as the reagent and also as the medium for dispersing the sodium.
Generally, the dispersion medium under consideration consists of one or more aromatic or saturated liquid hydrocarbons such as, for example, toluene, a xylene or n-octane.
Toluene is a particularly advantageous dispersion medium. In order especially to avoid the aggregation of the metal and so as to reduce the surface tension, one or more dispersants may be added, if necessary, during the stirring phase when the metal is at a temperature above its melting point.
Alternatively, these dispersants can be introduced into the dispersion medium before or simultaneously with the addition of the metal.
Dispersants that are generally used include higher fatty acids preferably containing at least 15 carbon atoms, such as, for example, oleic acid, higher fatty alcohols or esters of high molecular weight, these compounds preferably containing at least 15 carbon atoms.
A polymer such as polyethylene, which will especially have the effect of greatly increasing the stability of the dispersion medium, may also be used.
These dispersants will usually be used in a proportion of from 0.5% to 1% of the weight of metal used.
Moreover, the metallation reaction is carried out in the absence of water and under an inert atmosphere generally at a temperature that may range from 0xc2x0 C. to +40xc2x0 C., for example at a temperature from 0xc2x0 C. to +30xc2x0 C. and preferably at a temperature from about 0xc2x0 C. to +10xc2x0 C.
This reaction advantageously takes place in a solvent, preferably an ether such as, for example, tetrahydrofuran or dimethoxyethane, and preferably in the presence of a slight excess of thiophene, such as from 1.2 to 1.5 mol per mole of metal.
In addition, this metallation reaction is carried out in the presence of an agent capable of transferring a single electron between the metal and the thiophene. These are generally aliphatic or aromatic conjugated diene compounds that may be selected from the compounds of formula: 
in which:
R1 represents hydrogen or a methyl, ethyl or phenyl radical,
R2 represents hydrogen or a methyl or ethyl radical,
R3 represents a radical 
in which R4 and R5, which may be identical or different, represent hydrogen or a methyl or ethyl radical or R3 represents a phenyl, benzyl or 1-phenyl-1-ethyl group.
Among the compounds of formula I above that may be mentioned are 1,3-butadiene, 2-methyl-1,3-butadiene or isoprene, 1-phenylethylene or styrene, 1-methyl-1-phenylethylene or xcex1-methylstyrene, 1,1-phenylethylene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.
Isoprene, xcex1-methylstyrene and 2,3-dimethyl-1,3-butadiene represent single-electron-transfer agents that are particularly advantageous in the context of the invention. However, isoprene and better still xcex1-methylstyrene represent preferred conjugated dienes.
As regards the reaction with ethylene oxide, this preferably takes place in the thiophene metallation medium and at a temperature ranging from 0xc2x0 C. to room temperature, in particular at a temperature of about 20xc2x0 C.
Likewise, similar operating conditions may be observed for the hydrolysis reaction.